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How to Clean and Restore Limestone
Stone buildings are built to last, and for many centuries, limestone has been a popular exterior stone for buildings in the UK. Restoring limestone is easier as it is softer than other stones such as granite, but at the same time incredibly tough and durable. You only have to look at the stone churches and country houses across the UK. Hundreds of years after laying the first stones, the buildings are still going strong. However, it’s common for limestone buildings to need cleaning and restoration to keep them looking their best.
Why clean or restore limestone?
Limestone is a strong and natural rock that can stand the test of time. However, it’s very porous and can become marked, stained, and scratched relatively easily, especially when near heavily trafficked roads. Cleaning ensures that the limestone stays looking its best while also removing any unwanted paintwork and blends original limestone with an extension or repairs. Cleaning can also reveal the building’s true condition where dirt deposits could be hiding failed pointing, cracks, damaged stonework, or even structural faults.
Before limestone cleaning can commence, a survey is required to inspect the façade from top to bottom. This will provide crucial information on the building’s age, style, history and construction and identify the nature, pattern, and cause of the soiling. It will also determine whether soiling has caused damage and decay to the materials and checks for any areas that may need extensive repair. In some instances, the survey may reveal that it would be best not to completely remove the soiling as it could cause further unnecessary damage. However, it can also indicate that cleaning the limestone is necessary and will suggest which cleaning methods are most suitable and won’t damage the limestone.
Cleaning test trials
Cleaning test trials often accompany the pre-cleaning survey to determine the most effective and safest cleaning method to recommend. Cleaning test trials will usually begin with the gentlest cleaning options such as intermittent sprays of minimal water, poulticing and wet air abrasion. This is essential in ensuring the limestone is cleaned and restored in the most sympathetic way possible.
Cleaning limestone should always be carried out by a professional experienced in cleaning and restoring delicate stonework. The pre-cleaning survey results combined with the soiling on the limestone will determine the best cleaning method to use.
Depending on the degree of dirt and decay, you can clean limestone with a low-pressure spray of purified water, or “pure water cleaning”. This method avoids subjecting the limestone to the corrosive metals commonly found in water. Your solution should be selected based upon the results of your test trials: hot water, low-pressure, and steam is the best method to restore, preserve and maintain soiled building façades. After rinsing the limestone, it may be advisable to agitate the surface with a brush to remove any remaining dirt or debris. A pH-neutral cleaner can be used for tough areas if required. However, it’s important to avoid using harsh or acidic soaps which can greatly damaging the porous stone.
Another common method for cleaning limestone is poultice cleaning which is often used on heavily soiled limestone and sandstone surfaces. The soiling is softened or diluted by chemicals contained in the poultice and is drawn away from the surface, removing much of the soiling. The façade can then be washed down with a low pressure water wash. If necessary, the process can be repeated to achieve the best cleaning level.
The poultice can be applied by trowel and left to act for the right time, as determined via prior testing. This method allows a high degree of control over the duration of action and targeting the chemicals on detailed parts of the façade. In warm weather, it is recommended to apply a protective film of polythene to the poultice to prevent the product from drying out, but generally the product should be left for 24 hours and then removed, followed by a steam clean for enhanced results.
Other limestone cleaning methods sometimes used include:
- Rubbing and brushing, wet and dry abrasives or surface dressing
- Sponging, water sprays, and steam cleaning
- Organic solvents, acidic or alkaline treatments
- Laser cleaning and ultra-sonics
Some cleaning methods could damage the delicate limestone if not carried out correctly. Using too much water can also activate salts in the stone, leading to fixings and iron cramps corrosion while also damaging internal plasterwork, timber, or decoration. It can also cause the limestone to become susceptible to frost, causing the surface to shatter. Meanwhile, more abrasive methods can cause pitting of the surface and open the stone’s pores, leading to rapid re-soiling.
See our other blog, ‘choosing the best cleaning solution for your stone building‘, to find out more about how to safely clean and restore a stone building.
Restoring limestone buildings
With some listed stone buildings, if the limestone is only weathered but still structurally sound, it’s often preferred to leave the building as it is. However, there are times when the damage is simply too great to ignore, and you need to restore the limestone. Before beginning the task of restoring limestone, you need to assess the damage carefully. You need to look for chips, scratches, crumbling mortar, missing stones, and discolouration. Many of these issues can be dealt with by specialist repointing, cleaning, and patching techniques.
If there is extensive deterioration, such as large chunks of missing stones or large amounts of loose mortar, it’s a good idea seek the advice of a professional restorer. This will ensure you minimise the risk of causing further damage to the building and determine if there are any underlying structure or water drainage issues, causing the damage.
To maintain, restore and protect the limestone building well into the future, you need to select the most sympathetic cleaning and restoration solution. Our specialists have extensive knowledge and experience in cleaning and restoring limestone and the conservation of listed and heritage buildings. Speak to a member of our team today about restoring your limestone building.
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